The present invention relates to a process for treating a gas mixture comprising hydrogen, H2S and other components, such as hydrocarbons, with a view to separating the hydrogen from the other components of the mixture while maintaining it at the pressure of the initial gas mixture.
The present invention relates to the treatment of products coming from hydrotreatment (HT) processes very widely employed in the refining industry. Various types of hydrotreatment (HT) processes coexist in most refineries and are used for treating a large number of refining products, particularly the following cuts: gasoline, kerosene, diesel, vacuum distillates, oil bases. These hydrotreatment processes are used to adjust certain properties of the refining products, such as the contents of sulfur, nitrogen and aromatic compounds, or the cetane number. Sulfur is often the key property (the units are also often referred to as HDS (standing for hydrodesulfurization) units) and is the subject of increasingly stringent specifications leading refiners to seek ways of improving these units.
Chemical hydrogenation reactions take place in a reactor in which the hydrocarbon feed is mixed with a stream of hydrogen (in large excess) and passes over a catalyst bed. Some of the hydrogen reacts with the unsaturated organosulfur and organonitrogen compounds, producing hydrogen sulfide (H2S), ammonia (NH3), C1-C6 light hydrocarbons (HC) and saturated heavier compounds. On the downstream side of the reactor, a liquid/vapor separation tank is used to recover the hydrogen-rich gas phase, which is recycled in order to create this hydrogen excess (hereafter called the recycling gas). This gas contains, in addition to hydrogen, most of the volatile compounds that are created in the reactor and have a tendency to concentrate therein. The chemically consumed hydrogen, and also the hydrogen lost by mechanical losses, dissolution or purging is compensated for by a hydrogen-rich make-up gas, the composition of which varies depending on its mode of production. Typically, the hydrogen content of this gas (hereafter called the make-up gas) varies between 70 mol % and 99.9 mol %, the remainder generally being methane or a mixture of light hydrocarbons.
An essential parameter of the hydrotreatment reaction is the hydrogen partial pressure at the outlet of the reactor. This partial pressure depends on the total pressure of the unit (set during the “design” of the unit), on the degree of vaporization of the hydrocarbon feed (set by the total pressure and the operating temperature) and above all on the hydrogen concentration of the two gases—the make-up gas and the recycling gas—that are used. The H2S partial pressure is a second important parameter that depends mainly on the H2S content of the recycling gas, and therefore on the sulfur of the feed, and on the degree of desulfurization applied during the hydrotreatment. It is therefore desirable in hydrodesulfurization units to increase the hydrogen partial pressure and to reduce the H2S partial pressure by purifying either the make-up gas or the recycling gas, or both gases. The objective is therefore to reduce as far as possible the H2S and hydrocarbon contents.
The prior art has already proposed various solutions for achieving this objective. Thus, a first solution consists in purging the recycling gas in order to limit its H2S and hydrocarbon concentration: a fraction of the recycling gas is drawn off in order to remove the noncondensable gases that have built up in the recycling loop. These gases are discharged into what is called the “fuel gas” line; this line is present in all refineries and collects all the gaseous effluents that can be utilized in the form of energy. However, this high-pressure purge has several drawbacks:                the impact on the hydrogen and H2S partial pressures is slight;        since the recycling gas is rich in hydrogen, the primary consequence of the purge is the loss of hydrogen to the “fuel gas” line of the refinery. This hydrogen is then utilized since it is employed as fuel; and        because of this loss, a larger amount of make-up gas has to be introduced. However, the make-up gas is compressed in order to go from the pressure of the hydrogen line to the operating pressure of the unit. The high-pressure purge is therefore limited by the capacity of the make-up gas compressor.        
A second solution consists in employing a scrubbing step in which the recycling gas is scrubbed by an amine solution. During this scrubbing, H2S is completely absorbed and then desorbed by regeneration of the amine, and finally converted into liquid sulfur, for example in a Claus unit placed downstream. However, the scrubbing relates only to H2S and removes none of the hydrocarbons from the recycling gas. The impact on the H2S partial pressure is substantial, but the impact on the hydrogen partial pressure is negligible. The gain in hydrodesulfurization performance achieved thanks to this scrubbing therefore remains modest. In addition, the amine solutions pose corrosion and foaming problems.
A third solution of the prior art, which is very widely employed for H2/CO/CH4 mixtures, consists in purifying the hydrogen of the recycling gas by adsorption. This adsorption is used to achieve purity levels of higher than 99.5%. The application of adsorption to a hydrodesulfurization recycling gas is, for example, disclosed in JP 57055992. The adsorption treatment of the recycling gas or of the recycling gas mixture or of the make-up gas has substantial influence on the hydrodesulfurization performance. However, this solution has never been applied on an industrial scale to the treatment of these gases because of low yields. This is because the hydrogen yield of adsorption units is generally between 70 and 90%. The loss of hydrogen must therefore be compensated for by the use of a larger amount of make-up gas. The increase in the volume of make-up gas may be up to 100% in the case of the treatment of the entire recycling gas by a PSA (Pressure Swing Adsorption)-type process. The use of a PSA process therefore involves a high hydrogen cost; this is also greatly limited by the capacity of the make-up gas compressor and is in practice impossible without extensive investment.
A fourth solution of the prior art is the recovery of the hydrogen contained in the recycling gas by treating this gas using a hydrogen-permeable membrane. This type of membrane makes it possible to obtain high hydrogen purities (90 to 98%) and acceptable yields (80 to 98%, depending on the desired purity). The cost is moderate compared with the previous solutions. This solution has been described in the field of hydrodesulfurization units in EP-A-061 259. This solution is applied on an industrial scale, but the problem that remains is the need to recompress the recycling gas after it has passed through the membrane. This is because the purified hydrogen is produced at reduced pressure and the performance of the membrane is better the lower the production pressure. In practice, it is impossible to treat all the recycling gas. The membrane is therefore generally placed in a recycling gas branch and the hydrogen produced is sent to the intake of the make-up gas compressor in order to return to the pressure of the unit. The volume treated by the membrane, and consequently its effectiveness, is once again limited by the capacity of the make-up gas compressor.
A fifth solution is the use of reverse-selectivity membranes, which maintain the hydrogen under pressure. However, these membranes have low hydrogen/hydrocarbon selectivities (particularly in the case of hydrogen/methane separation). They therefore allow all of the recycling gas to be treated (as described in patents U.S. Pat. No. 6,190,540 and U.S. Pat. No. 6,179,996) in order to carry out more selective hydrocarbon purging, but a compromise between the loss of hydrogen and the degree of purification of the hydrogen has to be found. If the objective is the high purification of the recycling gas (with a hydrogen purity of 90%, or even 95%), then the hydrogen losses are very substantial (30% or 50%, or even higher) and the limitations are the same as those of a simple purge, (corresponding to the abovementioned first solution). If the objective is to reduce the hydrogen losses in comparison with a conventional high-pressure purge (as in the above first solution), then the purification and the impact on the hydrodesulfurization performance are very moderate. The last drawback is that the loss of hydrogen varies with the composition of the recycling gas to be treated—it is greater the richer the recycling gas to be treated is in hydrogen.
There is therefore a need to improve the hydrodesulfurization units of the prior art, and especially the treatment of the recycling gas and the use of this gas and of the make-up gas. The object of the invention is to propose a process for treating the gas coming from a hydrodesulfurization unit so as to obtain a recycling gas having a high hydrogen purity without reducing the pressure of the gas and without loss of hydrogen during this treatment.